Aging is associated with an increased occurrence of infection and cancer, and, as people age, they begin to exhibit age-related immune deficiencies, collectively termed immunosenescence. To determine the effects of age on human monocytes, 'aged monocytes' (isolated from individuals > or = 65 years of age) were compared with 'young monocytes' (isolated from individuals approximately 25 years of age) for their ability to be activated by lipopolysaccharide. Our results show that aged monocytes display a decrease in their cytotoxicity against tumor cells in vitro, a decrease in interleukin (IL1) secretion (although no decrease in IL1 precursor production was observed), a decrease in reactive oxygen and nitrogen intermediate (ROI/RNI) release, an increase in intracellular levels of cyclic adenosine monophosphate and a loss of protein kinase translocation. Therefore, aged monocytes present distinct characteristics of immunosenescence.